1. Field of the Invention
The invention is directed to a method for controlling the temperature of an intermittently standing and flowing fluid (a gaseous and/or liquid medium) which is at rest during standstill intervals and is flowing during flow intervals.
2. Background of the Art
The developer apparatus of a color proofing system is exemplary of the need to maintain an intermittently supplied fluid at a specific desired temperature during flow intervals. In such a device, the coated fluid is developed by wetting it with developer liquid. For this purpose, the proof is moved along a developer liquid dispenser unit.
Known developer apparatus for color proofing systems are provided with a tempering device for tempering the developer liquid, i.e., for controlling the temperature of the developer liquid. For obtaining proofs of high quality, it is usually required that the developer liquid supplied to the developer liquid dispenser unit has a temperature within a predetermined range of temperatures (generally 23.degree. C.-26.degree. C.). For obtaining multi-colored proofs, the above-described developing process is performed for each of the primary colors of the proof. Usually, multicolored proofs are composed of the three colors a) red, blue and yellow or b) cyan, magenta, and yellow, and black in addition. Thus, the development of such a proof is carried out in four developing processes, which are interrupted by the laminating of the corresponding color particles and the exposure of the proofs provided with the color particles. This means that, when generating a multi-color proof, only phase-wise use is made of the developer apparatus each time. For the quality of the multicolored proof, it is important that the developer liquid is kept at nearly an identical temperature during all development processes for a proof. In this regard, the absolute value of the temperature of the developer liquid is less important as long as the temperature is in the above-defined range. Known developer apparatus are equipped with a temperature sensor or the like for detecting the temperature of the developer liquid. A control unit connected to the temperature sensor will then control the tempering device correspondingly for maintaining the developer liquid at a substantially constant temperature.
A problem of temperature control resides in that the temperature of the developer liquid need be exact only during the time periods when the proof passes through the developer apparatus. In known developer apparatus, to safeguard this exactness, the complete supply quantity of the developer liquid is kept at the (optimum) operating temperature. This is economically disadvantageous because, as the case may be, there have to be tempered large quantities of developer liquid which nonetheless will not be used for the developing process.
In view of the above problem, an advantage is obtained by tempering only that quantity of developer liquid which is currently supplied. Such continuous-flow tempering devices are known (DE 33 40 667 A1, DE 40 36 210 A1, WO 89/05129, DE 41 09 677 A1, EP 0 338 283 A1). Considering that a developer apparatus for developing an exposed proof (DIN-A3) requires about 140 ml of a developer liquid to be supplied in about one minute and that this developer liquid has to be kept exactly on the desired temperature, it is evident that the demands posed on the accuracy of the operation of the temperature control are rather high. In particular, control deviations in the form of differences between the desired temperature and the actual temperature cannot be tolerated. Thus, the used control should include an integral portion which is responsible for the stationary accuracy. However, also a proportional portion should be provided which handles the dynamics (jump-like changes at the input of the control or disturbing factors). Because of the two completely different operating modes, "developer liquid flowing" and "developer liquid standing," temperature control in a continuous-flow tempering device is still not without difficulties.
Temperature control systems designed for flowing fluids which operate with integral, proportional and/or differential portions which can be switched in dependence from the respective operating mode, are known, e.g., from EP-A-0 303 833, U.S. Pat. No. 4,991,770 and EP-A-0 454 132. Further temperature control systems are disclosed in U.S. Pat. No. 4,055,296, U.S. Pat. No. 3,834,617 and EP-A-0 298 415. None of these control systems can be used in developer apparatus for color proofing systems because the temperature control is not performed with sufficient accuracy.